Matlab/Octave fftshift utility
Matlab and Octave have a simple utility called fftshift that performs this bin rotation. Consider the following example:
octave:4> fftshift([1 2 3 4]) ans = 3 4 1 2 octave:5>If the vector [1 2 3 4] is the output of a length 4 FFT, then the first element (1) is the dc term, and the third element (3) is the point at half the sampling rate (
), which can be taken to be
either plus or minus
since they are the same point on the
unit circle in the 
plane. Elements 2 and 4 are plus and minus
, respectively. After fftshift, element (3) is first,
which indicates that both Matlab and Octave regard the spectral sample
at half the sampling rate as a negative frequency. The next element
is 4, corresponding to frequency 
, followed by dc and
.
Another reasonable result would be fftshift([1 2 3 4]) == [4 1
2 3], which defines half the sampling rate as a positive frequency.
However, giving
to the negative frequencies balances giving dc
to the positive frequencies, and the number of samples on both sides
is then the same. For an odd-length DFT, there is no point at
, so the result
octave:4> fftshift([1 2 3]) ans = 3 1 2 octave:5>is the only reasonable answer, corresponding to frequencies
, respectively.
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